Thermionic cathodes and methods of making



2, 1961 v. L. STOUT 2,996,795

THERMIONIC CATHODES AND METHODS OF MAKING Filed June 28, 1955 Alkaline Earth Oxide Nio/r/e Titanium Point with Powdered Mixture of a Metal and an Active Material Selected from the Group Consisting ot Titanium Zirconium Tantalum and the Hydrides thereof Heat Painted Base to Alloy Powders Together and to Base App/y E mllssive Coating Mount in Tube Envelope and Process Inventor Virgil L .Stout His Attorney.

United States Patent 2,996,795 TI-IERMIO'NIC CA'IHODES AND NEETHODS OF MAKING Virgil L. Stout, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed June '28, *1955, Ser. 'No. 518,547 22 Claims. (Cl. 29195) The present invention relates to improved thermionic cathodes and methods of making and particularly to cathodes employing an electron emissive coating.

Certain uncoated metals, such as tungsten and tantalum, have been used substantially as electron emitters in electron discharge devices. These metals have a relatively high work function and must be raised to a high temperature to produce adequate electron emission. For this reason, uncoated cathodes have been limited in their use essentially to high power-type devices. Tungsten is also very diflicult to shape mechanically and accordingly its use has been limited to filamentary-type cathodes.

Cathodes employing the so-called oxide coatings have also been Widely used. These cathodes are formed by applying a coating including a carbonate or mixture of carbonates selected from the group consisting of barium carbonate, strontium carbonate and calcium carbonate. The carbonates are applied to a suitable metal base, frequently nickel, and reduced to an oxide during the manufacture of the device in which they are incorporated. It is generally assumed that the actual thermionic emission of these cathodes during operation depends upon the existence of free metal resulting from a further reduction of the oxides. While these oxides provide emission at relatively low operating temperatures, for example, in the order of 630 C. to 1030 C., they are subject to certain disadvantages. One of the most serious of these is the sensitiveness to contamination or poisoning of the cathode which results from the presence of gases enclosed within the vacuum envelope. It is also difiicult with cathodes employing these coatings to obtain large emission current densities over a reasonable operating life. Extensive work has been done in attempting to improve cathodes of this type and they have found wide use in ordinary receiving tubes. The disadvantages mentioned above, however, exist to a serious degree and it is an object of the present invention to improve the emitting and life characteristics of cathodes employing such coatings. A generally similar type of coated cathode uses a thorium oxide coating as the emissive layer. The present invention is applicable to cathodes having coatings of this type, consideration being given to the higher operating temperatures encountered as compared with cathodes having an alkaline earth oxide coating.

In accordance with an important aspect of my invention, an intermediate spongy or porous alloy layer is alloyed to the base and the emissive coating is infiltrated into and formed as a layer over this porous alloy. One of the constituent metals of the alloy is relatively active with respect to the emissive coating and provides a means for reducing the oxide coating during operation of the device. This active metal also tends to absorb any gas that may be liberated from the base metal or other- Wise during the operation of the tube so as to minimize the possibility of poisoning the emissive cathode. The porous nature of the intermediate layer greatly extends the area of contact between the alloy and the oxide coating and enhances the ability of the layer to perform its reducing and gas-absorbing functions and at the same time greatly improves the mechanical adherence of the oxide coating to the cathode.

Further objects and adantages of my invention will be better understood by reference to the following de- Ice scription of specific examples thereof and its scope will be pointed out in the appended claims.

As illustrated in the accompanying block diagram, my invention may be carried out by coating a suitably clean metal cathode base with a mixture of powdered materials which alloy together and to the cathode base. One of the materials of the powdered mixture is selected from the group consisting of titanium, zirconium, tantalum and the hydrides thereof, and the other metal, one which does not fully melt during the alloying process. The porous layer resulting is intimately bonded to the base and forms an extensive surface to which the emissive coating is applied. The coating may be one of the alkaline earth carbonates, such as barium, calcium or strontium carbonate or a mixture thereof. It is preferably applied by forming the carbonate into a paste or liquid by mixing with a suitabe binder and painting or spraying onto the porous alloy coating. The emissive coating may likewise be formed in a similar manner with thorium oxide as the basic emissive material.

The active material of the alloy layer serves to reduce the oxide or carbonate of the coating and to establish a relatively high level of emission, consistent with reasonable operating life Without the usual difficulties encountered in activating an oxide-type cathode. In addition the active metal absorbs gas during operation of the tube and precludes the possibility of later poisoning of the cathode by gases liberated during operation and resultant loss of emission from the cathode.

The base metals may be selected from those in the group frequently used as cathode base materials and including, for example, nickel, iron, platinum, tungsten and tantalum. The first three, namely nickel, iron and platinum, are particularly suitable for a base material where the alkaline earth carbonate type of coating is employed since the operating temperatures of the cathode will not be excessive for these metals. While tungsten and tantalum can also be used, their high temperature properties are not essential and the tendency is to use one of the first three metals mentioned. On the other hand, with the thorium oxide emissive coating, the operating temperature of the cathode is somewhat higher and accordingly tungsten or tantalum base is used as a base metal.

In the preparation of the alloy layer, the metal in addition to the active material may be selected from the same group as the base material and in any given instance may be either the same or different from the metal of the cathode base. For example, if an alkaline earth oxide coating is used, iron, nickel and platinum powders are preferable. One of these is mixed in a suitable binder with a suitable amount of a powder of the active metal or compound, such as titanium, zirconium or tantalum and the hydrides thereof. In practice, titanium, zirconium and the hydn'des thereof are preferable as the active ingredients in cathodes employing an alkaline earth oxide type of emissive coating.

For the thorium oxide-coated cathodes which operate at a substantially higher temperature, for example, in the range of 1400 C. to 1750 C., it is desirable to use the more refractory metals as the base material and accordingly tungsten and tantalum are preferred. The porous coating is formed by a mixture of powders, such as tungsten, for example, with either tantalum or zirconium or the hydrides thereof. For these cathodes,

than is desirable iron-titanium, platinum-titanium, nickel-zirconium and iron-zirconium, it being understood that the titanium and zirconium may be originally in the form of a corresponding hydride which is reduced during the alloying process.

In accordance with a specific example 'of my invention applied to a cathode utilizing an alkaline earth oxide coating, I utilize a nickel base which is painted with a mixture of nickel and titanium hydride powders in a nitrocellulose binder. The exact percentage of the titanium hydride used may be varied over a considerable range and a mixture including 95% nickel and titanium hydride has been used with success. The coated nickel base is next vacuum-fired at a temperature of about 1300 C. during which firing, the hydrogen of the hydride is released and the nickel and titanium powder alloyed to the nickel base and together. The alloying of the powders is at the surface of the nickel particles and the resultant alloy surrounding the remaining solid nickel particles has a composition dependent upon the temperature of the vacuum firing. In the particular example given, the temperature was maintained at 1300 C. for about five minutes and a resultant alloy at the surface of the nickel particles of about 90% nickel and titanium was formed. The heating is preferably accomplished by high frequency induction and with this method the cathode and particularly the powdered metals reach their ultimate temperature very rapidly. As a result, the firing temperature may be reachedin a very short time, such as one minute. The five minutes used give a large safety factor and insures that the metals have reached their final temperature. The alloy sponge was then coated with the alkaline earth carbonates by painting on a liquid formed by mixing the carbonate powders in a suitable binder.

I have shown in the drawing an elevational view in section illustrating schematically a portion of a cathode produced in accordance with the above specific example of my invention. As indicated by the legends, the cathode includes a nickel base, an intermediate porous layer of nickel titanium alloy and an outer layer of the emissive oxide coating which fills the pores of the alloy layer. The pores of the alloy layer are maintained since the nickel powder is not completely alloyed as the central portions of the nickel particles remain solid during the heating step.

In the above specific example, a mixture of powders consisting essentially of 95 nickel powder and 5% titanium hydride powder was used. For this specific composition, a firing temperature of 1300 C. is satisfactory since it is high enough to melt the contacting areas of the powdered materials where the percentage of titanium is higher, but insufficient to melt all of the nickel powder. The temperature used determines the final composition of the alloyed powders and may be readily selected from the phase diagram for the particular alloy involved. It is essential that it is high enough to obtain some liquid alloy and low enough to prevent melting of all of one of the powders, in this particular example, nickel.

Cathodes have been made with a substantial variation in the portion of the active material used in forming the alloy layer. For example, the percentage of titanium hydride in the nickel-titanium hydride mixture has been varied from about 2% to about 30% and the nickel cor- 'respondingly varied from about 98% to 70%. For a 2% titanium hydride mixture, a firing temperature of 1300 C. is satisfactory. For a 30% titanium hydride- 70% nickel powder mixture, a firing temperature of 1050 C. for five minutes gave a satisfactory alloying of the powders and to the base without melting all of the nickel particles. From the phase diagram for titanium and nickel, it will be apparent that the firing temperature must be between 1015 C. and 1110 C. In this way, the porous nature of the alloy layer is preserved and at the same time, the adherence of the alloy layer to the base is assured. The phase diagram for nickel-titanium gives the basic information for the firing temperature, it being understood that the actual amount of titanium available for alloying with the metal powder is reduced somewhat by the alloying with the nickel base.

Zirconium hydride powder may be substituted for the titanium hydride powder in making the porous alloy layer. remain substantially the same as those employed for titanium hydride.

Instead of using a nickel-titanium hydride or nickelzirconium hydride mixture, the nickel may be replaced with iron powder. In this case, the firing temperatures for a given percentage of the hydride are slightly higher. For example, for an iron-titanium hydride mixture of 95% iron and 5% hydride, the firing temperature is about 1320 C. When an iron-zirconium hydride mixture is employed, a percentage as low as about 1% zirconium hydride may be used, in which case the firing temperature is about 1350 C.

sive coating of thorium oxide, a tungsten base is alloyed with a tungsten-zirconium interlayer on which the thorium oxide is applied. In this case, the tungsten metal base is painted with a mixture of tungsten powder and zirconium hydride in a nitrocellulose binder with the percentage of tungsten approximately 87% by weight and the zirconium hydride approximately 13% by weight. The coated base is vacuum-fired at a temperature of about 1800 C. with the hydride being dissociated during the heating to form a tungsten-zirconium alloy at the surface of the tungsten base and over the surfaces of the tungsten particles. The liquid alloy at the surface of the tungsten particles is maintained during the heating and the alloyed portion reaches a composition of about 80% tungsten and 20% zirconium. Only the surface portion of the tungsten particles is melted with the remaining solid portions serving to maintain the porous nature of the alloy layer. Again, the emissive coating, in this case thorium oxide, is painted on the alloy sponge after the vacuum firing.

In the above specific example, the powder mixture from which the alloy layer is formed is 87% by weight of tungsten power and 13 by weight of zirconium hydride powder. The percentage of zirconium hydride power that may be employed may be varied between about 2% and 2 0% by weight and the tungsten powder correspondingly varied between 98% and Over this range of percentages, the limits of the firing temperature, taken from the phase diagram for zirconium and tungsten, remain the same, namely 1650 C. as a lower limit and 2175 C. as an upper limit. The temperature of 1800 C., used in the previous example, is satisfactory. It is apparent that the actual firing temperature with different starting percentages of tungsten Will determine the amount of the tungsten powder that remains unmelted.

Instead of the tungsten-zirconium mixture, a tungstentant-alum or tantalum hydride mixture can be employed. A tantalum base is also satisfactory in which case a tungsten-zirconium hydride powder mixture for the alloy layer is preferred.

In the above specific examples, the active metal has been used in the form of a hydride. While the hydrogen released from the hydride during the firing operation assures that the metals are free from oxide and readily alloyed, the process may be carried out by starting with the metal powders instead of the hydrides, particularly if the process is carried out under non-oxidizing conditions, such as in a vacuum. 7

It will be appreciated that the completed cathodes are then mounted in the discharge device in which they are I to be used and further processed in accordance with the usual practice for evacuating the electric discharge device. The cathodes, however, do not require activation Usable percentages and the firing temperatures in the same manner as conventional oxide-coated cathodes in that the active metal of the porous interlayer serves to reduce the oxide coating and provide the free metal which is generally considered to be the source of electron emission.

From the foregoing, it is clear that my invention provides a composite cathode structure and method of making it in which an intermediate porous alloy layer is alloyed to the base structure and includes a constituent metal which is active with respect to the emissive coating. These cathodes are not subject to the tendency of other coated cathodes to peel ofi from the base structure, and are also less subject to loss of emission during operating life due to the presence of the active metal in intimate contact with the emissive coating over an extended area. This active metal tends to absorb any gases which might otherwise tend to combine with the free metal of the emissive layer. A higher level of emission is obtained without the usual activation requirements without any significant decrease in operating life. Many of the oathodes tested have exhibited a longer life at higher emission levels than obtained with the same type of oxide on a conventional supporting base structure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermionic cathode comprising a metal base, a porous layer of metal particles alloyed together and to said base by a metal selected from the group consisting of titanium, zirconium and tantalum and an electron emissive coating impregnating and overlying said porous 6 said base and said mixture above the alloying tempera ture of said first and second ingredients and above the alloying temperature of said first ingredient with said metal base to form a porous layer of the alloy which is alloyed to said base, and applying to said porous layer an emission enhancing coating.

10. The method of making a composite cathode including a nickel base which comprises applying to an area of said base a powder mixture consisting essentially of nickel and a substance selected from the group consisting of titanium and titanium hydride, heating said base and said mixture above the alloying temperature of titanium and nickel to alloy said powders together and to said base without melting all of said nickel powder to form a porous alloy layer and applying to said porous layer an allkaline earth oxide type ocE emissive coating.

11. The method of making a composite cathode including a nickel base which comprises applying to an area of said base a powder mixture consisting essentially of nickel porous layer ofnickel particles alloyed together and to said base with zirconium, and an alkaline earth oxide electron emissive coating impregnating and overlying said porous layer.

5. A thermionic cathode comprising a nickel base, a porous layer of iron particles alloyed together and to said base with zirconium, and an alkaline earth oxide electron timissive coating impregnating and overlying said porous ayer.

6. A thermionic cathode comprising a tungsten base, a porous layer of tungsten particles alloyed together and to said base by zirconium and a thorium oxide electron emissive coating impregnating and overlying said porous layer.

7. A thermionic cathode comprising a tungsten base, a porous layer of tungsten particles alloyed together and to said base by tantalum and a thorium oxide electron Tmissive coating impregnating and overlying said porous ayer.

8. A thermionic cathode comprising a tantalum base, a porous layer of tungsten particles alloyed together and to said base by zirconium and a thorium oxide electron emissive coating impregnating and overlying said porous layer.

9. The method of making a composite cathode including a metal base member which comprises applying to an area of said base member a powder mixture having an alloying temperature below the melting point of the metal of said base member, said mixture consisting essentially of a first ingredient selected from the group consisting of titanium, zirconium, tantalum and the hydrides thereof and a second ingredient selected from the group consisting 0t iron, nickel, platinum and tungsten, heating and a substance selected from the group consisting of zirconium and zirconium hydride, heating said base and said mixture above the alloying temperature of zirconium and nickel to alloy said powders together and to said base without melting all of said nickel powder to form a porous alloy layer and applying to said porous layer an alkaline earth oxide type of emissive coating.

12. The method of making a composite cathode including a nickel base which comprises applying to an area of said base a powder mixture consisting essentially of iron and a substance selected from the group consisting of titanium and titanium hydride, heating said base and said mixture above the alloying temperature of titanium and iron to alloy said powders together and to said base without melting all of said iron powder to form a porous alloy layer and applying to said porous layer an alkaline earth oxide type of emissive coating.

13. The method of making a composite cathode including a nickel base which comprises applying to an area of said base a powder mixture consisting essentially of iron and a substance selected from the group consisting of zirconium and zirconium hydride, heating said base and said mixture above the alloying temperature of zirconium and iron to alloy said powders together and to said base without melting all of said iron powder to form a porous alloy layer and applying to said porous layer an alkaline earth oxide type of emissive coating.

14. The method of making a composite cathode including a tungsten base which comprises applying to an area of said base a powder mixture consisting essentially of tungsten and a substance selected from the group consisting of zirconium and zirconium hydride, heating said base and said mixture above the alloying temperature of zirconium and tungsten to alloy said powders together and to said base without melting all of said tungsten powder to form a porous alloy layer and applying to said porous layer a thorium oxide emissive coating.

15. The method of making a composite cathode including a tungsten base which comprises applying to an area of said base a powder mixture consisting essentially of tungsten and a substance selected from the group consisting of tantalum and tantalum hydride, heating said base and said mixture above the alloying temperature of tantalum and tungsten to alloy said powders together and to said base without melting all of said tungsten owder to form a porous alloy layer and applying to said porous layer a thorium oxide emissive coating.

16. The method of making a composite cathode including a tantalum base which comprises applying to an area of said base a powder mixture consisting essentially of tungsten and a substance selected from the group con sisting of zirconium and zirconium hydride, heating said base and said mixture above the alloying temperature of zirconium and tungsten to alloy said powders together and to said base without melting all of said tungsten powder to form a porous, alloy layer and applying to said porous layer a thorium oxide emissive coating. T

17. The method of making a composite cathode including abase member of nickel, applying to said base member a mixture of powder particles consisting essentially of nickel and a substance selected from the group consisting of titanium and titanium hydride and nickel, said nickel constituting 70 to 98% by weight of said mixture of powders, heating said base and applied layer in vacuum to a temperature in the range of 950 C. to 1300 C. to melt portions of said powder particles contacting one another and said base to alloy said powder particles together and to said base, and applying an alkaline earth compound type of emissive material to said porous alloy layer to form an emissive coating thereon.

18. The method of making a composite cathode including a base member of nickel, applying to said base member a mixture of powder particles consisting essentially of nickel and a substance selected from the group consisting of titanium and titanium hydride and nickel, said nickel constituting about 95% by weight of said mixture of powders, heating said base and applied layer in vacuum to a temperature of about 1300" C. to melt portions of said powder particles contacting one another and said base to alloy said powder particles together and to said base, and applying an alkaline earth compound type of emissive material to said porous alloy layer to form an emissive coating thereon.

19. The method of making a composite cathode including a base member of tungsten, applying to said base member a mixture of powder particles consisting essentially of tungsten and a substance selected from the group consisting of zirconium and zirconium hydride, heating said base and applied layer in vacuum to a temperature in the range 1650 C. to 2175 C. to melt portions of said powder panticles contacting one another and said base member to alloy said powder particles together and to said base, and applying thorium oxide to said porous alloy layer to fill the pores of said alloy layer to form a coating thereon.

20. The method of making a composite cathode including a base member of tungsten, applying to said base member a mixture of powder particles consisting essentially of tungsten and a substance selected from the group consisting of zirconium and zirconiumhydride, said tungsten powder constituting approximately 87% by weight of said mixture, heating said base and applied layer in vacuum to a temperature of about 1800 C. to melt portions of said powder particles contacting one another and said base member to alloy said powder particles together and to said base, and applying thorium oxide to said porousjalloy layer to fill the pores of said alloy layer to form a coating thereon. 9 Y

21. The method of making a' composite "cathode including a metal base member which comprises applying to an area of the base member a powder mixture having an alloying temperature below the melting point of the metal of the base member, the powder mixture consisting essentially of a first ingredient selected from the group consisting of titanium and zirconium and the hydrides thereof, and a second ingredient selected from the group consisting of iron, nickel and platinum, heating the base and mixture to alloy the ingredients together and to the base to form a porous layer, and applying an emission enhancing coating to the porous layer.

22. The method of making a composite cathode including a metal base selected from the materials of tungsten and tantalum which comprises applying to an area of the base member a powder mixture consisting essentially of a first ingredient selected from the group consisting of zirconium and tantalum and the hydrides thereof and a second ingredient selected from the group consisting of tungsten and tantalum, heating the base and mixture to alloy the ingredients together and to the base to form a porous layer, and applying an emission enhancing coating to the porous layer.

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1. A THERMIONIC CATHODE COMPRISING A METAL BASE, A POROUS LAYER OF METAL PARTICLES ALLOYED TOGETHER AND TO SAID BASE BY A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND TANTALUM AND AN ELECTRON EMISSIVE COATING IMPREGNATING AND OVERLYING SAID POROUS LAYER. 